Recent mobile communication has developed rapidly. Among others, mobile phones have proliferated outstandingly and improvements have been made to reduce their size and weight significantly. According to mobile phone standards, two particular frequency bands are used respectively in different regions: in Japan, a 800 MHz band and a 1.5 GHz band for Personal Digital Cellular (PDC); in Europe, a 900 MHz band and a 1.9 GHz band for Global System for Mobile Communications (GSM); and in U.S., a 800 MHz band for Advanced Mobile Phone System (AMPS) and a 1.9 GHz band for Personal Communications System (PCS). Moreover, communication systems such as Global Positioning System (GPS) using 1.5 GHz, Bluetooth using a 2.4 GHz band, and International Mobile Telecommunications (IMT) 2000 using a 2 GHz band are put in practical use for mobile communication and data transmission. If a single antenna is capable of operating in the above-mentioned multiple frequency bands, it would be ideal for the purpose of reducing antenna size and weight.
Furthermore, there is a plan in progress to adopt the GSM that has been used in Europe in U.S. as a mobile phone scheme so that a same mobile phone can be used in U.S. and Europe. However, the GSM in Europe uses a band of 880 to 960 MHz and a band of 1710 to 1880 MHz, whereas the GSM in U.S. is designed to use a band of 824 to 894 MHz and a band of 1850 to 1990 MHz. An antenna capable of operating in the frequency bands in both Europe and U.S. is required to cover both a wide frequency band of 136 MHz ranging from 824 to 960 MHz and a wide frequency band of 280 MHz ranging from 1710 to 1990 MHz.
So far, a single antenna capable of operating in the above multiple frequency bands has not existed. So far, an antenna covering the wide frequency bands so it can operate in the GSM frequency bands in both U.S. and Europe has not existed.
By the way, antennas with reduced size and weight for use in mobile phones have been proposed in Japanese Patent Application Laid-Open (JP-A) No. 2001-284935 and Japanese Patent Application Laid-Open (JP-A) No. 2002-43826. The principles of these techniques will be briefly described below. FIG. 26 shows a basic structure of an antenna of prior art, wherein one end of an antenna element 10 is connected to a feeding point 12 and the other end thereof is electrically connected to a ground conductor 14. The most part of the antenna element 10 is straightened in approximately parallel with the ground conductor 14 except the upright ends for the connections to the feeding point 12 and the ground conductor 14. The entire electrical length of the antenna element 10 is set to ½ wavelength (λ/2) or 1 wavelength (λ) of a frequency band in which the antenna operates. Moreover, the antenna element maybe formed in a coil or meandering pattern or appropriately bent into a loop for size reduction purposes. These techniques can be used for only a single frequency band. In FIG. 26, a dotted line denotes current distribution.
FIG. 27 shows another prior art antenna, wherein a capacitor 16 is inserted in series in the center of the antenna element 10 of prior art shown in FIG. 26. The electrical length of the antenna element plus the capacitor 16 is set to ½ wavelength of a frequency band in which the antenna operates. Current distribution denoted by a dotted line in FIG. 27 indicates that an in-phase current is produced in the antenna element 10 and this is effective particularly for a case where antenna directivity is important.
FIG. 28 shows yet another prior art antenna, wherein the capacitor 16 is inserted at a point on the antenna element 10, nearer to the feeding point 12, not in the center, as a modification to the prior art antenna shown in FIG. 27. FIG. 29 shows yet another prior art antenna, wherein two parallel conductors 28 which are disconnected in direct current are inserted in series between the ends of the antenna element 10. The two parallel conductors 18 are inductively coupled together and function as a single antenna element as a whole.
FIG. 30 shows a further prior art antenna, wherein a matching circuit 20 is inserted between one end of the antenna element 10 and the feeding point and the other end of the antenna element 10 is electrically connected to the ground conductor 14. In the prior art antenna shown in FIG. 30, the length of the antenna element 10 is not required to be ½ wavelength of a frequency band in which the antenna operates. The antenna element 10 and the matching circuit 20 should be set appropriately so that the electrical length containing the antenna element 10 and the matching circuit 20 will be ½ wavelength.
However, any antenna of the above prior art is designed to operate in a single frequency band and cannot operate in multiple frequency bands. Thus, a mobile phone that uses two frequency bands needs two antennas for different frequency bands. A mobile communication device in which a plurality of communication systems including GPS are installed needs a plurality of antennas. Hence, it is difficult to reduce the size and weight of a mobile communication device by using any of the above prior art antennas.
It is therefore an object of the present invention, which has been made in view of the above circumstances of prior art, to provide an antenna for multiple bands employing an single antenna element 10, the antenna being capable of operating in multiple frequency bands and ideal for size and weight reduction purposes.